Gradient impedance high-temperature ceramic wave-absorbing material based on 3D printing technology and preparation method thereof

Abstract

The application discloses a gradient impedance high-temperature ceramic wave-absorbing material based on a 3D printing technology and a preparation method thereof, and belongs to the technical field of wave-absorbing materials. The selective laser sintering technology is adopted to print a composite powder into a ceramic body with a gradient pore structure, and the ceramic body is debound. The debound ceramic body is placed in a ceramic precursor solution, vacuum pressure impregnation is carried out, and then precursor conversion ceramic material is obtained through further cross-linking, solidification and pyrolysis. The precursor conversion ceramic material is subjected to the above-mentioned impregnation-cross-linking-solidification process for 1-8 times, and a ceramic wave-absorbing material preform is obtained. The ceramic wave-absorbing material preform is placed in a low-dielectric nano-material precursor sol, vacuum pressure impregnation is carried out, and then heat treatment is further carried out to convert the impregnated sol into inorganic nano-material, so that the gradient impedance high-temperature ceramic wave-absorbing material based on the 3D printing technology with good wave-absorbing performance and high-temperature resistance is prepared.

Description

Technical Field

[0001] This invention belongs to the field of microwave absorbing materials technology, specifically relating to a gradient impedance high-temperature ceramic microwave absorbing material based on 3D printing technology and its preparation method. Background Technology

[0002] Radar stealth technology is a key technical indicator for the next generation of hypersonic aircraft. Since the surface temperature of the aircraft exceeds 1000°C at ultra-high speeds, it is prone to airflow burn-out problems, which puts forward higher requirements for the stable service of radar absorbing materials at high temperatures.

[0003] Resin-based microwave absorbing materials are a widely researched topic in the current technology. For example, Chinese patent document CN112266593A discloses a biodegradable bioresin-based microwave absorbing material and its preparation method. This invention uses polylactic acid or polybutylene succinate as the resin matrix and adds ceramic matrix, iron oxide, and nano-graphene as absorbers in a reasonable ratio to expand the absorption band range of electromagnetic waves and enhance the absorption rate. Chinese patent document CN103333465A discloses a preparation method of FeCo@MWNTs / epoxy resin-based microwave absorbing composite material. This invention uses a liquid-phase blending method to obtain iron and cobalt nanoparticles modified carbon nanotubes, disperses the iron and cobalt nanoparticles modified carbon nanotubes in an epoxy resin matrix, uses ethylenediamine as a curing agent, and after ultrasonication and vacuuming to eliminate bubbles, it is cast into a mold to prepare the iron and cobalt nanoparticles modified carbon nanotube / epoxy resin microwave absorbing composite material.

[0004] However, the aforementioned traditional material preparation techniques are difficult to use to prepare high-temperature ceramic microwave absorbing materials with complex structures, and the forming accuracy is relatively low, making it difficult to maintain the stability of the material's microwave absorption performance. The material's temperature resistance and mechanical properties also need to be further improved.

[0005] Selective laser sintering (SLS), also known as selective laser sintering, is a 3D printing technology based on additive manufacturing. Its basic working principle involves adding material layer by layer via laser sintering, building up the material layer by layer to form the final product. This method offers advantages such as simple manufacturing process, high forming accuracy, high material utilization, and short production cycle. Ceramic preforms formed by SLS have a porous structure, which not only improves the impedance matching between the ceramic material and air but also increases the transmission path of electromagnetic waves within the absorbing structure, thus promoting electromagnetic wave attenuation. This makes it a promising technology for the preparation of composite microwave absorbing materials. Summary of the Invention

[0006] This invention provides a method for preparing gradient impedance high-temperature ceramic microwave absorbing materials based on 3D printing technology. This method has high molding precision, wide material applicability, no need for mold support, and can prepare complex structural components. The resulting high-temperature ceramic microwave absorbing materials have good microwave absorption performance and high temperature resistance.

[0007] The specific technical solution adopted is as follows:

[0008] A method for preparing a gradient impedance high-temperature ceramic microwave absorbing material based on 3D printing technology includes the following steps:

[0009] S01 uses selective laser sintering technology to print composite powder into a ceramic green body with a gradient pore structure, and then degreases the ceramic green body; the composite powder includes low dielectric loss ceramic powder, binder and additives, the mass ratio of low dielectric loss ceramic powder to binder is 1:0.1-0.2, and the additives are hollow sphere ceramic powder and / or microwave absorbing agent.

[0010] S02 The degreased ceramic green body obtained in step S01 is placed in a ceramic precursor solution, and after vacuum pressure impregnation, it is further crosslinked, cured, and pyrolyzed to obtain the precursor-converted ceramic material; the ceramic precursor solution includes organosilicon material, crosslinking agent and organic solvent in a mass ratio of 1:0.3-0.5:0.4-1.2.

[0011] S03 involves repeating the infiltration-crosslinking curing-pyrolysis process of step S02 1-8 times to convert the precursor into ceramic material, thereby obtaining a ceramic-based microwave absorbing material preform.

[0012] S04 places the ceramic-based microwave absorbing material preform in a low-dielectric nanomaterial precursor sol, and after vacuum pressure impregnation, further heat treatment transforms the impregnated sol into inorganic nanomaterials to prepare the gradient impedance high-temperature ceramic microwave absorbing material based on 3D printing technology; the low-dielectric nanomaterial precursor sol includes alumina nano sol, silica sol, titanium dioxide sol, or aluminum dihydrogen phosphate sol.

[0013] This invention develops a gradient impedance periodic structure high-temperature ceramic microwave absorbing material by integrating the design of dielectric loss materials and macroscopic structural electromagnetic loss. While fully utilizing material loss, the invention designs macroscopic structural parameters to give the material ultra-wideband electromagnetic absorption characteristics. On one hand, the invention modifies the composition and microstructure of the material through 3D printing composite powder and precursor infiltration processes, thereby controlling the dielectric constant of the microwave absorbing material from the perspective of material properties. On the other hand, it employs a gradient impedance structural design to improve the impedance matching of the material and increase the loss of electromagnetic waves. For example, gradient irregular honeycomb structures or gradient three-period minimal curved surface structures can increase the transmission path of electromagnetic waves, thereby enhancing electromagnetic wave loss. The edge scattering and resonance effects generated by the periodic multi-layered stepped structure can enhance electromagnetic wave energy loss.

[0014] Specifically, in the selective laser sintering (SLS) process, the 3D printing model is adjusted to give the printed ceramic green body a gradient pore structure; or, the mass percentages of additives and low-dielectric-loss ceramic powder in the composite powder system are varied in a gradient, increasing or decreasing sequentially, to give the printed ceramic green body a gradient pore structure. This invention obtains ceramic green bodies with gradient pore structures by printing different composite powders at different thicknesses during the SLS process or by adjusting the structural gradient (adjusting the 3D printing model).

[0015] Preferably, the hollow sphere ceramic powder is added at a rate of 0.1–20 wt.% of the low dielectric loss ceramic powder; the microwave absorbing agent includes a carbon-based microwave absorbing agent or a silicon carbide-based microwave absorbing agent. When the microwave absorbing agent is a carbon-based microwave absorbing agent, the amount added is 0.1–5 wt.% of the low dielectric loss ceramic powder; when the microwave absorbing agent is a silicon carbide-based microwave absorbing agent, the amount added is 0.1–30 wt.% of the low dielectric loss ceramic powder. Silicon carbide has a high density and small volume, so it can be added in larger quantities.

[0016] Preferably, the hollow sphere ceramic powder comprises alumina hollow spheres or silicon nitride hollow spheres, with an average particle size of 50-150 μm.

[0017] Preferably, the carbon-based microwave absorbing agent is at least one of carbon black, nanoporous carbon, nanographite powder, nanoredox graphite powder, chopped carbon fiber, and carbon nanotubes; the silicon carbide-based microwave absorbing agent is at least one of silicon carbide nanopowder, silicon carbide whiskers, and silicon carbide nanowires.

[0018] Preferably, the adhesive comprises epoxy resin or phenolic resin with an average particle size of 0.5-3 μm.

[0019] Preferably, the low dielectric loss ceramic powder is at least one of silicon nitride and aluminum nitride, with an average particle size of 10-150 μm.

[0020] The specific process of printing composite powder into ceramic green bodies using selective laser sintering technology is as follows: the composite powder is evenly spread on the processing platform and heated to the processing temperature. The laser emits a laser beam, and then the control program controls the laser to scan the processing platform according to the two-dimensional layers. After the laser beam scan is completed, the thickness of the powder layer is lowered, and printing continues until the set layer height is reached, thus obtaining a ceramic green body with a gradient pore structure.

[0021] In the material gradient design process, the first type of composite powder is first evenly spread on the processing platform and heated to the processing temperature. The laser emits a laser beam, and then the control program controls the laser to scan the processing platform according to the two-dimensional layers. After the laser beam scan is completed, the powder layer thickness is lowered and printing continues until the set layer height is reached. The same steps are then followed to print the second type of composite powder, the third type of composite powder, and so on, until a ceramic green body with a gradient pore structure is obtained.

[0022] Further preferred, the parameters for selective laser sintering technology are set as follows: scanning rate 2000-2600 mm / min, layer thickness 0.1-0.2 mm, and laser power 4-10 W.

[0023] Preferably, in step S01, the degreasing process is carried out under an inert protective gas atmosphere. The parameters of the degreasing process are as follows: the temperature is raised from room temperature to 250-350°C at a heating rate of 1-5°C / min, and then the temperature is further raised to 700-1000°C at a heating rate of 2-3°C / min. The temperature is held for 40-60 minutes, and then cooled to room temperature in the furnace to complete the degreasing process.

[0024] The purpose of step S02 is to introduce a silicon-based ceramic matrix into the porous preform printed by SLS through precursor infiltration. The main purpose is to enhance and densify the ceramic preform. At the same time, the ceramic converted from the introduced precursor also has the function of a microwave absorbing agent. If the printed composite powder does not contain a microwave absorbing agent, the ceramic converted from the precursor can also serve as a microwave absorbing phase.

[0025] Preferably, in step S02, the ceramic precursor solution further includes an organometallic compound; the mass ratio of the organometallic compound, organosilicon material, crosslinking agent and organic solvent is 0.01-0.3:1:0.3-0.5:0.4-1.2.

[0026] The organometallic compounds include, but are not limited to, aluminum acetylacetonate, zirconium acetylacetonate, iron acetylacetonate, tetrabutyl titanate, etc.; the organosilicon materials include, but are not limited to, polycarbosilane, polysiloxane, polysilazane, etc.; the crosslinking agent is divinylbenzene; the organic solvent includes xylene or tetrahydrofuran, etc.

[0027] Preferably, in step S02, when no organometallic compound is added to the ceramic precursor solution, the crosslinking curing parameters are as follows: under vacuum, the temperature is raised from room temperature to 150-200°C at a heating rate of 1-10°C / min, held for 250-350 min, and then cooled to room temperature to complete the crosslinking curing; when an organometallic compound is added to the ceramic precursor solution, the crosslinking curing parameters are as follows: under vacuum, the temperature is raised from room temperature to 100-150°C at a heating rate of 1-10°C / min, held for 60-120 min, and then quickly transferred to an atmospheric pressure reaction device. Under inert protective gas conditions, the temperature is raised to 280-400°C at a rate of 0.5-3°C / min, held for 120-180 min, and then cooled to room temperature to complete the crosslinking curing.

[0028] Preferably, in step S02, the pyrolysis process is carried out under an inert protective gas atmosphere. The parameters of the pyrolysis process are as follows: the temperature is raised from room temperature to 250-350℃ at a heating rate of 3-5℃ / min, and then the temperature is further raised to 1000-1200℃ at a heating rate of 8-12℃ / min, held at that temperature for 90-180min, and then cooled to room temperature with the furnace to complete the pyrolysis process.

[0029] The repeated impregnation-crosslinking curing-pyrolysis process in step S03 can increase the content of the introduced silicon-based ceramic matrix.

[0030] Preferably, in step S04, the heat treatment process is carried out in an inert protective gas atmosphere, and the parameters of the heat treatment process are as follows: the temperature is raised from room temperature to 250-350℃ at a heating rate of 3-5℃ / min, then the temperature is further raised to 1000-1200℃ at a heating rate of 8-12℃ / min, held for 20-40min, and then the temperature is raised to 1400-1600℃ at a rate of 3-5℃ / min, held for 90-150min; and then cooled to room temperature in the furnace to complete the heat treatment process.

[0031] The present invention also provides a method for preparing gradient impedance high-temperature ceramic absorbing material based on 3D printing technology, and the resulting material is a gradient impedance high-temperature ceramic absorbing material based on 3D printing technology.

[0032] The shape of the gradient impedance high-temperature ceramic absorbing material based on 3D printing technology can be customized, including but not limited to honeycomb structure, irregular honeycomb structure, lattice structure, lattice sandwich structure, multi-layer gradient structure, etc., which can meet the needs of different application scenarios.

[0033] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0034] (1) The preparation method provided by the present invention has the advantages of wide material applicability and the ability to form large-size complex structures, which can meet the requirements of material-structure integrated design and molding. The gradient impedance high-temperature ceramic microwave absorbing material based on 3D printing technology has a gradient impedance periodic structure, good microwave absorption performance, and can effectively absorb electromagnetic waves in the range of 8-18GHz. Moreover, the microwave absorption performance is stable and can withstand high temperature of 1400℃, which can meet the needs of practical use.

[0035] (2) The preparation method provided by the present invention has high molding precision, which solves the problem of unstable wave absorption performance caused by low molding precision in traditional processes.

[0036] (3) The gradient impedance periodic structure of the high-temperature ceramic absorbing material prepared by the method of the present invention has good designability and can be flexibly changed according to actual application requirements. The processing cycle of the high-temperature ceramic absorbing material is short and the cost is low. Attached Figure Description

[0037] Figure 1 Here are schematic diagrams and reflectivity curves of the gradient impedance high-temperature ceramic absorbing material based on 3D printing technology prepared in Example 1, where (a) is a schematic diagram of the model and (b) is a reflectivity curve.

[0038] Figure 2 Here are schematic diagrams and reflectivity curves of the gradient impedance high-temperature ceramic absorbing material based on 3D printing technology obtained in Example 2, where (a) is a schematic diagram of the model and (b) is a reflectivity curve.

[0039] Figure 3 The figures show a schematic diagram and a reflectivity curve of the gradient impedance high-temperature ceramic absorbing material based on 3D printing technology prepared in Example 3, where (a) is a schematic diagram of the model and (b) is a reflectivity curve. Detailed Implementation

[0040] The present invention will be further illustrated below with reference to the embodiments and accompanying drawings. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention.

[0041] Example 1

[0042] (1) Preparation of ceramic green body

[0043] Four g of hollow silicon nitride spheres with an average particle size of 100 μm were selected as pore-forming agents, four g of nanoporous carbon was selected as microwave absorbers, 40 g of epoxy resin with an average particle size of 0.5 μm was selected as binders, and 400 g of silicon nitride with an average particle size of 100 μm was selected as low dielectric loss ceramic powder. That is, the mass ratio of hollow sphere ceramic powder, microwave absorber, binder and low dielectric loss ceramic powder was 0.01:0.01:0.1:1, and the first composite powder was obtained after being fully mixed evenly.

[0044] Eight g of hollow silicon nitride spheres with an average particle size of 100 μm were selected as pore-forming agents, four g of nanoporous carbon was selected as microwave absorbers, 40 g of epoxy resin with an average particle size of 0.5 μm was selected as binders, and 400 g of silicon nitride with an average particle size of 100 μm was selected as low dielectric loss ceramic powder. That is, the mass ratio of hollow sphere ceramic powder, microwave absorber, binder and low dielectric loss ceramic powder was 0.02:0.01:0.1:1, and the second composite powder was obtained after being fully mixed.

[0045] 12g of hollow silicon nitride spheres with an average particle size of 100μm were selected as pore-forming agents, 4g of nanoporous carbon was selected as microwave absorbers, 40g of epoxy resin with an average particle size of 0.5μm was selected as binders, and 400g of silicon nitride with an average particle size of 100μm was selected as low dielectric loss ceramic powder; that is, the mass ratio of hollow sphere ceramic powder, microwave absorber, binder and low dielectric loss ceramic powder was 0.03:0.01:0.1:1, and the third composite powder was obtained after being fully mixed evenly.

[0046] Selective laser sintering (SLS) technology is used to print composite powder into ceramic blanks: The first type of composite powder is evenly spread on the processing platform and heated to the processing temperature. The laser emits a laser beam, and the control program controls the laser to scan the processing platform according to two-dimensional layers. After the laser beam scan is completed, the powder layer thickness is lowered, and printing continues until the set layer height is reached. The same steps are followed to print the second and third types of composite powder sequentially to the set layer height, thus obtaining a ceramic blank with a gradient pore structure. The printing parameters are set as follows: scanning rate 2300 mm / min, layer thickness 0.15 mm, and laser power 5 W.

[0047] (2) Degreasing

[0048] The ceramic blank was placed in a high-temperature atmosphere sintering furnace. After the vacuum was evacuated to 10 Pa, an inert protective gas N2 was introduced. The temperature was increased from room temperature to 300°C at a rate of 5°C / min, and then increased to 700°C at a rate of 2°C / min. The temperature was held at 700°C for 60 min, and then cooled to room temperature with the furnace.

[0049] (3) Infiltration

[0050] Preparation of ceramic precursor solution: Dissolve 200g of polycarbosilane in a mixed solution of 80g of divinylbenzene and 80g of tetrahydrofuran to obtain solution A; dissolve 4g of tetrabutyl titanate in a solution of 20g of tetrahydrofuran to obtain solution B; mix solution A and solution B and stir thoroughly to obtain ceramic precursor solution (that is, in the ceramic precursor solution, the mass ratio of organometallic compound, organosilicon material, crosslinking agent and organic solvent is 0.02:1:0.4:0.5).

[0051] The degreased ceramic green body is completely immersed in the ceramic precursor solution, and the vacuum degree is maintained at -0.1MPa for 20 minutes. The pressure is then adjusted to 0.2MPa and maintained for 20 minutes. The above steps are repeated twice until no obvious bubbles escape from the solution. The impregnation process is then complete, and the ceramic green body to be crosslinked is obtained.

[0052] (4) Crosslinking curing

[0053] The ceramic green body to be crosslinked obtained in step (3) is placed in a high-temperature programmed heating furnace. Under vacuum, the temperature is increased from room temperature to 100°C at a rate of 2°C / min and held for 120 min. Then, the ceramic green body is quickly transferred to an atmospheric pressure reaction device. Under the protection of high-purity nitrogen, the temperature is increased to 280°C at a rate of 1°C / min and held for 120 min. Then, it is cooled to room temperature to complete the crosslinking and curing.

[0054] (5) Pyrolysis

[0055] The ceramic blank to be pyrolyzed obtained in step (4) is placed in a high-temperature atmosphere sintering furnace for pyrolysis. The pyrolysis process is as follows: after the vacuum degree is evacuated to 10 Pa, an inert protective gas N2 is introduced and the temperature is increased from room temperature to 300℃ at a heating rate of 5℃ / min; the temperature is then increased to 1200℃ at a heating rate of 10℃ / min and held for 120 min. The furnace is then cooled to room temperature to complete the pyrolysis process.

[0056] (6) Repeated impregnation-crosslinking curing-pyrolysis

[0057] Repeat steps (3)-(5) three times to obtain a ceramic-based microwave absorbing material preform.

[0058] (7) Sol impregnation

[0059] The ceramic-based microwave absorbing material preform was impregnated in aluminum dihydrogen phosphate sol. The impregnation process was as follows: maintain a vacuum of -0.1 MPa for 20 min; adjust the pressure to 0.2 MPa and maintain for 20 min; repeat twice until no obvious bubbles escape from the solution, thus completing the sol impregnation process.

[0060] (8) Heat treatment

[0061] The ceramic-based microwave absorbing material preform obtained by sol impregnation in step (7) was placed in a high-temperature atmosphere sintering furnace for heat treatment. The heat treatment process was as follows: the vacuum was evacuated to 10 Pa and then filled with high-purity nitrogen. The temperature was increased from room temperature to 300°C at a heating rate of 5°C / min. The temperature was then increased to 1200°C at a heating rate of 10°C / min and held for 30 min. The temperature was then increased to 1600°C at a heating rate of 5°C / min and held for 120 min. The temperature was then cooled to room temperature with the furnace to complete the heat treatment process, so that the impregnated sol was converted into aluminum phosphate, and the gradient impedance high-temperature ceramic microwave absorbing material based on 3D printing technology was prepared.

[0062] The model diagram of the gradient impedance high-temperature ceramic absorbing material based on 3D printing technology obtained in this embodiment is shown below. Figure 1 As shown in (a) of Figure 1, the reflectivity curves tested using the bow-shaped method are shown in (b) of Figure 1. In this high-temperature ceramic absorbing material, the porous carbon and SiC and free carbon phases generated by the conversion of precursors serve as absorbing functional phases, giving it good electromagnetic loss capability. The gradient porous structure design not only improves the impedance matching characteristics of the material, but also increases the transmission path of electromagnetic waves. Under the synergistic effect of the two, this high-temperature ceramic absorbing material exhibits excellent absorption performance, with a reflection loss of less than -10dB in the range of 2-4.2GHz and 10-18GHz, and an effective absorption bandwidth of 10.2GHz.

[0063] Example 2

[0064] (1) Preparation of ceramic green body

[0065] Four g of hollow alumina spheres with an average particle size of 100 μm were selected as pore-forming agents, eight g of silicon carbide whiskers were selected as microwave absorbers, 40 g of epoxy resin with an average particle size of 0.5 μm was selected as binders, and 400 g of silicon nitride with an average particle size of 100 μm was selected as low dielectric loss ceramic powder. That is, the mass ratio of hollow sphere ceramic powder, microwave absorber, binder and low dielectric loss ceramic powder was 0.01:0.02:0.1:1, and the first composite powder was obtained after being fully mixed.

[0066] Eight g of hollow alumina spheres with an average particle size of 100 μm were selected as pore-forming agents, eight g of silicon carbide whiskers were selected as microwave absorbers, 40 g of epoxy resin with an average particle size of 0.5 μm was selected as binders, and 400 g of silicon nitride with an average particle size of 100 μm was selected as low dielectric loss ceramic powder. That is, the mass ratio of hollow sphere ceramic powder, microwave absorber, binder and low dielectric loss ceramic powder was 0.02:0.02:0.1:1, and the second composite powder was obtained after being fully mixed.

[0067] 12g of hollow alumina spheres with an average particle size of 100μm were selected as pore-forming agents, 8g of silicon carbide whiskers were selected as microwave absorbers, 40g of epoxy resin with an average particle size of 0.5μm was selected as binders, and 400g of silicon nitride with an average particle size of 100μm was selected as low dielectric loss ceramic powder; that is, the mass ratio of hollow sphere ceramic powder, microwave absorber, binder and low dielectric loss ceramic powder was 0.03:0.02:0.1:1, and the third composite powder was obtained after being fully mixed evenly.

[0068] Selective laser sintering (SLS) technology is used to print composite powder into ceramic blanks: The first type of composite powder is evenly spread on the processing platform and heated to the processing temperature. The laser emits a laser beam, and the control program controls the laser to scan the processing platform according to two-dimensional layers. After the laser beam scan is completed, the powder layer thickness is lowered, and printing continues until the set layer height is reached. The same steps are followed to print the second and third types of composite powder sequentially to the set layer height, thus obtaining a ceramic blank with a gradient pore structure. The printing parameters are set as follows: scanning rate 2300 mm / min, layer thickness 0.15 mm, and laser power 5 W.

[0069] (2) Degreasing

[0070] The ceramic blank was placed in a high-temperature atmosphere sintering furnace. After the vacuum was evacuated to 10 Pa, an inert protective gas N2 was introduced. The temperature was increased from room temperature to 300°C at a rate of 5°C / min, and then increased to 700°C at a rate of 2°C / min. The temperature was held at 700°C for 60 min, and then cooled to room temperature with the furnace.

[0071] (3) Infiltration

[0072] Preparation of ceramic precursor solution: Dissolve 200g of polycarbosilane in a mixed solution of 80g of divinylbenzene and 80g of tetrahydrofuran to obtain solution A; dissolve 4g of aluminum acetylacetonate in a solution of 20g of tetrahydrofuran to obtain solution B; mix solution A and solution B and stir thoroughly to obtain the ceramic precursor solution (that is, the mass ratio of organometallic compound, organosilicon material, crosslinking agent and organic solvent in the total ceramic precursor solution is 0.02:1:0.4:0.5).

[0073] The degreased ceramic green body is completely immersed in the ceramic precursor solution, and the vacuum degree is maintained at -0.1MPa for 20 minutes. The pressure is then adjusted to 0.2MPa and maintained for 20 minutes. The above steps are repeated twice until no obvious bubbles escape from the solution. The impregnation process is then complete, and the ceramic green body to be crosslinked is obtained.

[0074] (4) Crosslinking curing

[0075] The ceramic green body to be crosslinked obtained in step (3) is placed in a high-temperature programmed heating furnace. Under vacuum, the temperature is increased from room temperature to 100°C at a rate of 2°C / min and held for 120 min. Then, the ceramic green body is quickly transferred to an atmospheric pressure reaction device. Under the protection of high-purity nitrogen, the temperature is increased to 320°C at a rate of 1°C / min and held for 120 min. Then, it is cooled to room temperature to complete the crosslinking and curing.

[0076] (5) Pyrolysis

[0077] The ceramic blank to be pyrolyzed obtained in step (4) is placed in a high-temperature atmosphere sintering furnace for pyrolysis. The pyrolysis process is as follows: after the vacuum degree is evacuated to 10 Pa, an inert protective gas N2 is introduced and the temperature is increased from room temperature to 300℃ at a heating rate of 5℃ / min; the temperature is then increased to 1200℃ at a heating rate of 10℃ / min and held for 120 min. The furnace is then cooled to room temperature to complete the pyrolysis process.

[0078] (6) Repeated impregnation-crosslinking curing-pyrolysis

[0079] Repeat steps (3)-(5) three times to obtain a ceramic-based microwave absorbing material preform.

[0080] (7) Sol impregnation

[0081] The ceramic-based microwave absorbing material preform was impregnated in aluminum dihydrogen phosphate sol. The impregnation process was as follows: maintain a vacuum of -0.1 MPa for 20 min; adjust the pressure to 0.2 MPa and maintain for 20 min; repeat twice until no obvious bubbles escape from the solution, thus completing the sol impregnation process.

[0082] (8) Heat treatment

[0083] The ceramic-based microwave absorbing material preform obtained by sol impregnation in step (7) was placed in a high-temperature atmosphere sintering furnace for heat treatment. The heat treatment process was as follows: the vacuum was evacuated to 10 Pa and then filled with high-purity nitrogen. The temperature was increased from room temperature to 300°C at a heating rate of 5°C / min. The temperature was then increased to 1200°C at a heating rate of 10°C / min and held for 30 min. The temperature was then increased to 1600°C at a heating rate of 5°C / min and held for 120 min. The temperature was then cooled to room temperature with the furnace to complete the heat treatment process, so that the impregnated sol was converted into aluminum phosphate, and the gradient impedance high-temperature ceramic microwave absorbing material based on 3D printing technology was prepared.

[0084] The model diagram of the gradient impedance high-temperature ceramic absorbing material based on 3D printing technology obtained in this embodiment is shown below. Figure 2As shown in (a), the reflectivity curves measured using the bow-shaped method are shown in (b). In this high-temperature ceramic absorbing material, the polarization effect generated by the numerous holes and structural defects produced by silicon carbide whiskers and aluminum doping SiC under the action of an electric field enhances the polarization loss of electromagnetic waves. The numerous heterogeneous interfaces in the material enhance the interfacial polarization loss. On the other hand, the oblique honeycomb structure design causes multiple reflections and scattering of incident electromagnetic waves within the material, which also enhances the material's electromagnetic loss capability. Therefore, this high-temperature ceramic absorbing material exhibits excellent absorption performance, with a reflection loss of less than -10dB in the range of 2.5-6GHz and 10.1-18GHz, and an effective absorption bandwidth of 11.4GHz.

[0085] Example 3

[0086] (1) Preparation of ceramic green body

[0087] 8g of silicon carbide whiskers were selected as the microwave absorber, 40g of epoxy resin with an average particle size of 0.5μm was selected as the binder, and 400g of silicon nitride with an average particle size of 100μm was selected as the low dielectric loss ceramic powder; that is, the mass ratio of microwave absorber, binder and low dielectric loss ceramic powder was 0.02:0.1:1, and the composite powder was obtained after being fully mixed evenly.

[0088] Selective laser sintering (SLS) technology is used to print composite powder into ceramic blanks: the composite powder is evenly spread on the processing platform and heated to the processing temperature. The laser emits a laser beam, and the control program controls the laser to scan the processing platform according to two-dimensional layers. After the laser beam scan is completed, the thickness of one powder layer is lowered, and printing continues. During this process, the 3D printing model is adjusted to make the printed ceramic blank have a gradient hole structure until the set layer height is reached, resulting in a ceramic blank with a gradient hole structure. The printing parameters are set as follows: scanning rate 2300 mm / min, layer thickness 0.15 mm, and laser power 5 W.

[0089] (2) Degreasing

[0090] The ceramic blank was placed in a high-temperature atmosphere sintering furnace. After the vacuum was evacuated to 10 Pa, an inert protective gas N2 was introduced. The temperature was increased from room temperature to 300°C at a rate of 5°C / min, and then increased to 700°C at a rate of 2°C / min. The temperature was held at 700°C for 60 min, and then cooled to room temperature with the furnace.

[0091] (3) Infiltration

[0092] Preparation of ceramic precursor solution: Dissolve 200g of polycarbosilane in a mixed solution of 80g of divinylbenzene and 100g of tetrahydrofuran to obtain ceramic precursor solution (that is, in the ceramic precursor solution, the mass ratio of organosilicon material, crosslinking agent and organic solvent is 1:0.4:0.5).

[0093] The degreased ceramic green body is completely immersed in the ceramic precursor solution, and the vacuum degree is maintained at -0.1MPa for 20 minutes. The pressure is then adjusted to 0.2MPa and maintained for 20 minutes. The above steps are repeated twice until no obvious bubbles escape from the solution. The impregnation process is then complete, and the ceramic green body to be crosslinked is obtained.

[0094] (4) Crosslinking curing

[0095] The ceramic preform to be crosslinked obtained in step (3) is placed in a high-temperature programmed heating furnace. Under vacuum, the temperature is increased from room temperature to 180°C at a rate of 2°C / min and held for 300 min. Then it is cooled to room temperature to complete the crosslinking and curing.

[0096] (5) Pyrolysis

[0097] The ceramic blank to be pyrolyzed obtained in step (4) is placed in a high-temperature atmosphere sintering furnace for pyrolysis. The pyrolysis process is as follows: after the vacuum degree is evacuated to 10 Pa, an inert protective gas N2 is introduced and the temperature is increased from room temperature to 300℃ at a heating rate of 5℃ / min; the temperature is then increased to 1200℃ at a heating rate of 10℃ / min and held for 120 min. The furnace is then cooled to room temperature to complete the pyrolysis process.

[0098] (6) Repeated impregnation-crosslinking curing-pyrolysis

[0099] Repeat steps (3)-(5) four times to obtain a ceramic-based microwave absorbing material preform.

[0100] (7) Sol impregnation

[0101] The ceramic-based microwave absorbing material preform was impregnated in aluminum dihydrogen phosphate sol. The impregnation process was as follows: maintain a vacuum of -0.1 MPa for 20 min; adjust the pressure to 0.2 MPa and maintain for 20 min; repeat twice until no obvious bubbles escape from the solution, thus completing the sol impregnation process.

[0102] (8) Heat treatment

[0103] The ceramic-based microwave absorbing material preform obtained by sol impregnation in step (7) was placed in a high-temperature atmosphere sintering furnace for heat treatment. The heat treatment process was as follows: the vacuum was evacuated to 10 Pa and then filled with high-purity nitrogen. The temperature was increased from room temperature to 300°C at a heating rate of 5°C / min. The temperature was then increased to 1200°C at a heating rate of 10°C / min and held for 30 min. The temperature was then increased to 1600°C at a heating rate of 5°C / min and held for 120 min. The temperature was then cooled to room temperature with the furnace to complete the heat treatment process, so that the impregnated sol was converted into aluminum phosphate, and the gradient impedance high-temperature ceramic microwave absorbing material based on 3D printing technology was prepared.

[0104] The model diagram of the gradient impedance high-temperature ceramic absorbing material based on 3D printing technology obtained in this embodiment is shown below. Figure 3 As shown in (a), the reflectivity curves measured using the bow-shaped method are shown in (b). In this high-temperature ceramic absorbing material, the SiC generated from silicon carbide whiskers and precursors undergoes dipole polarization loss under the action of an electric field. The free carbon phase in the precursor enhances the material's conductivity loss. Furthermore, the porous surface of the three-period minimal curved structure increases the transmission path of electromagnetic waves and improves the material's impedance. Therefore, this high-temperature ceramic absorbing material exhibits excellent absorption performance, with a reflection loss of less than -10dB in the 8.5-18GHz range and an effective absorption bandwidth of 9.5GHz.

[0105] The embodiments described above provide a detailed explanation of the technical solutions of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, additions, or similar substitutions made within the scope of the principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a gradient impedance high-temperature ceramic wave-absorbing material based on 3D printing technology, characterized in that, It comprises the following steps: S01: printing a ceramic green body with gradient pore structure by using selective laser sintering technology, and debinding the ceramic green body; the composite powder comprises low dielectric loss ceramic powder, binder and additive, the mass ratio of low dielectric loss ceramic powder and binder is 1:0.1-0.2, the additive is hollow sphere ceramic powder and wave absorber; the binder is epoxy resin or phenolic resin; S02: placing the debound ceramic green body obtained in step S01 in a ceramic precursor solution, and after vacuum pressure impregnation, further cross-linking and curing and pyrolysis, a precursor transformed ceramic material is obtained; the ceramic precursor solution comprises organic silicon material, cross-linking agent and organic solvent with a mass ratio of 1:0.3-0.5:0.4-1.2; S03: repeating the impregnation-cross-linking and curing-pyrolysis process of step S02 for 1-8 times to obtain a ceramic-based wave-absorbing material preform; S04: placing the ceramic-based wave-absorbing material preform in a low dielectric nano-material precursor sol, and after vacuum pressure impregnation, further heat treatment is performed to convert the impregnated sol into inorganic nano-material, thereby preparing the gradient impedance high-temperature ceramic wave-absorbing material based on 3D printing technology; the low dielectric nano-material precursor sol is aluminum dihydrogen phosphate sol; In step S02, the ceramic precursor solution further comprises a metal organic compound; the mass ratio of the metal organic compound, organic silicon material, cross-linking agent and organic solvent is 0.01-0.3:1:0.3-0.5:0.4-1.2; the metal organic compound is aluminum acetylacetonate, zirconium acetylacetonate, iron acetylacetonate or tetrabutyl titanate; the organic silicon material is polycarbosilane, polysilaborazane or polysilazane; the cross-linking agent is divinylbenzene; and the organic solvent is xylene or tetrahydrofuran; In step S04, the heat treatment process is carried out in an inert protective gas atmosphere, and the parameters of the heat treatment process are as follows: the temperature is raised from room temperature to 250-350℃ at a rate of 3-5℃ / min, then the temperature is further raised to 1000-1200℃ at a rate of 8-12℃ / min, and the temperature is kept for 20-40min, then the temperature is continuously raised to 1400-1600℃ at a rate of 3-5℃ / min, and the temperature is kept for 90-150min; and the heat treatment process is completed after cooling to room temperature.

2. The preparation method of the gradient impedance high-temperature ceramic wave-absorbing material based on 3D printing technology according to claim 1, characterized in that, in the selective laser sintering process, the 3D printing model is adjusted so that the ceramic green body obtained by printing has a gradient pore structure; or the mass percentage of the additive and the low dielectric loss ceramic powder in the composite powder system is gradiently changed, and is sequentially increased or decreased, so that the ceramic green body obtained by printing has a gradient pore structure.

3. The preparation method of the gradient impedance high-temperature ceramic wave-absorbing material based on 3D printing technology according to claim 1, characterized in that, The hollow sphere ceramic powder is added in an amount of 0.1-20 wt.% of the low dielectric loss ceramic powder; the wave absorber includes a carbon-based wave absorber or a silicon carbide-based wave absorber, when the wave absorber is the carbon-based wave absorber, the wave absorber is added in an amount of 0.1-5 wt.% of the low dielectric loss ceramic powder; when the wave absorber is the silicon carbide-based wave absorber, the wave absorber is added in an amount of 0.1-30 wt.% of the low dielectric loss ceramic powder.

4. The method for preparing the gradient impedance high-temperature ceramic wave-absorbing material based on the 3D printing technology according to claim 3, characterized in that, The hollow sphere ceramic powder includes alumina hollow spheres or silicon nitride hollow spheres, and the average particle size is 50-150 μm; The carbon-based wave absorber is at least one of carbon black, nano-porous carbon, nano-graphite powder, nano-oxidation-reduction graphite powder, short carbon fibers and carbon nanotubes; and the silicon carbide-based wave absorber is at least one of silicon carbide nano-powder, silicon carbide whiskers and silicon carbide nanowires. The average particle size of the binder is 0.5-3 μm. The low dielectric loss ceramic powder is at least one of silicon nitride and aluminum nitride, and the average particle size is 10-150 μm. 5.The method for preparing a gradient impedance high-temperature ceramic wave-absorbing material based on 3D printing technology according to claim 1, characterized in that, In step S02, The parameters for cross-linking and curing are as follows: in a vacuum environment, the temperature is raised from room temperature to 100-150 °C at a rate of 1-10 °C / min, and then the temperature is quickly transferred to a normal-pressure reaction device under the condition of inert protective gas, the temperature is raised to 280-400 °C at a rate of 0.5-3 °C / min, and the temperature is kept for 120-180 min, and then the temperature is cooled to room temperature, and the cross-linking and curing are completed. 6.The method for preparing a gradient impedance high-temperature ceramic wave-absorbing material based on a 3D printing technology according to claim 1, characterized in that, In step S02, The pyrolysis process is carried out in an inert protective gas atmosphere, and the parameters for the pyrolysis process are as follows: the temperature is raised from room temperature to 250-350 °C at a rate of 3-5 °C / min, and then the temperature is further raised to 1000-1200 °C at a rate of 8-12 °C / min, the temperature is kept for 90-180 min, and then the temperature is cooled to room temperature, and the pyrolysis process is completed.

7. The gradient impedance high-temperature ceramic wave-absorbing material based on the 3D printing technology prepared by the method for preparing the gradient impedance high-temperature ceramic wave-absorbing material based on the 3D printing technology according to any one of claims 1-6.

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